On-line dirt counter

ABSTRACT

The two-dimensional medium-pass filter principle is used to acquire a sharpened image initially In practice the sharpened image is obtained from the difference of a linear array signal of a partially out-of-focus image from another linear array signal of the same image but in focus. Integrated intensity of dirt specks higher than a user selected threshold are measured and accumulated. Each integrated intensity value is converted into its corresponding speck size with the use of a predefined fined look-up table. The counter generates a dirt speck histogram and statistical data within a preselected time interval or area coverage.

TECHNICAL FIELD

The invention relates to a dirt counter for measuring and recording dirtspeck sizes and intensities on paper. More specifically, the inventionrelates to an on-line dirt counter which measures the dirt speck sizesand intensities on a surface of a web of moving paper.

BACKGROUND ART

Several off-line dirt counters have been developed. A typical one, suchas the BIOTRAN™ (of New Brunswick Scientific) extracts measurements suchas total area and total counts from two-dimensional incoming images.Recently, an off-line system PAPRICAN MICROSCANNER™ (of Noram QualityControl and Research Equipment Ltd.) image analyzer applies themedium-pass filter as well as visual impact principles to detect andmeasure dirt speck size of two-dimensional images. The INTEC™ (of IntecCorp.) on-line dirt counter uses a laser light source and fiber opticreceivers. It can detect dirt specks as small as 0.05 mm². However, thisdevice is not designed to measure dirt speck size using visual impactprinciples similar to human judgment as defined in TAPPI (TechnicalAssociation of the Pulp and Paper Industry) count.

There are also issued patents of interest which deal with the detectionof flaws on surfaces. U.S. Pat. No. 4,172,666, Clarke, Oct. 30, 1979distinguishes between different types of faults detected on a movingweb. A defect detector scans the surface of the web with a laser andviews the reflected light with a view or several detectors at variousangles. Sums and differences between signals are used to discern whichdefects scatter light and which absorb it. The signal processing is notspecifically related to the emulation of visual inspection as is thepresent invention.

U.S. Pat. Nos. 4,794,264 and 4,794,265, Quackenbos et al, Dec. 27, 1988,relate to an apparatus and method for uniquely detecting pits on asmooth surface. The moving surface is illuminated with a point source oflaser light through a pair of beam splitters. Flaws and pits reflectspecularly with different patterns. An annular mask shields one detectorto view the neighborhood of a focal point and a window on the otherdetector constrains the signal to the central focus.

The invention in the above patents uses a beam splitter to view the samespot with a pair of detectors and to difference a central spot signalwith a wider surround. However, Quackenbos et al illuminate the samplewith a laser through the same beam splitter in order to observe glossyspecular reflectance but does not illuminate diffusely outside the beamsplitter to observe the fuse reflectance. Further, Quackenbos et al usea single detector in each beam rather than a linear array of detectors,and Quackenbos et al mask the two detectors to see only their intendeddomains of view rather than having the two beams differ only insharpness of focus. Finally, Quackenbos et al use the difference ofdetector signals to distinguish flaws from pits but does not have asystem wherein the signal differences correspond to a sharpeningoperation.

U.S. Pat. No. 4,740,079, Koizumi et al, Apr. 26, 1988, teaches a methodand apparatus for detecting foreign substances, e.g. dirt specks orsmudges, on the glossy surface of a semi-conductor chip which alreadycontains delivered etches of the circuit. In the Koizumi et al patent, acircuit component is illuminated at near grazing incidence with a laseror a pair of lasers as the chip is physically moved past the detectionconfiguration. A beam splitter divides the View of the surface of thechip in two identical beams with different polarization. Each beam isviewed at the same focus by a linear array of detectors. The differencesignal corresponds to the diffused scattering from dirt on the chip.However, Koizumi et al does not calculate the integrated optical densityas a visual impact parameter.

U.S. Pat. No. 4,724,481, Nishioka, Feb. 9, 1988, teaches a flaw detectorfor detecting a flaw in a sheet. The apparatus includes an array cameraor series of array cameras which view a moving sheet. A circuit employsan electronic shading correction to enhance the signal to noise ratio. Amemory associated with each photodetection element is used to maintain areference level for that element. The novelty of this patent appears toreside in the algorithm of shading correction and data reduction.However, the Nishioka invention cannot be used on paper sheets withrealistic reflectance nonuniformity associated with formation.

U.S. Pat. No. 4,237,539, Piovoso et al, Dec. 2, 1980, teaches an on-lineweb inspection system which includes a transverse web scanning means.X-ray film is made at high speeds and must be inspected for defects atspeeds of up to 900 feet per minute. In the '539 patent, a flying spotcrisscrosses crosses the sheet with a scan repeat of 1.5 mm. Thisrequires three scans per millisecond. A sophisticated processing boardin the host computer processes the signal and sorts features with sharpedges by size. In the '539 patent, a scanning laser is used rather thana diode array. In addition, the '539 patent does not use a board levelprocessing of the signal to extract and sort the images of the defects.Instead, sorting is accomplished by area and by the sharpness of theedge of the feature, for example, rather than by integration of thesignal strength within the bounds of a spot.

A system for detecting and classifying flaws on metallic surfaces istaught in U.S. Pat. No. 4,253,768, Yaroshuk et al, Mar. 3, 1981. In the'768 patent, a scanning laser illuminates a moving surface of, forexample, a pipe, which is viewed by at least two detectors. Thereflectance angles for the detectors are chosen so that most of thelight reflects into the first detector when there are no defects with alow but finite intensity reaching the other detectors. The otherdetectors are positioned to receive increased intensity when the laserbeam traverses a defect. The average signal serves as a base line forcomparisons. This system has, of course, very little in common withsystems for detecting dirt specks on a sheet of paper, and especiallythe system herein.

A particle detection method and system, for scanning the surface of asemi-conducting wafer, is taught in U.S. Pat. No. 4,766,324, Saadat etal, Aug. 23, 1988. The surfaces scanned by a laser beam, and thescattered light indicates the presence of a dirt speck. The central ideaof this patent is a procedure for identification of dirt that wasalready present before the current processing step, and to distinguishthis from dirt added by the most recent step. Once again, this haslittle in common with a dirt counter for measuring dirt speck sizes on aweb of moving paper.

U.S. Pat. No. 4,665,317, Ferriere et al, May 12, 1987, teaches a processand equipment for sensing surface defects on a moving strip of rolledmetal. Successive video images of a moving sheet of the metal arepresented to a computer memory as a continuous representation of thesurface. Two-dimensional digital filters are used for contour detectionand the edge detection of defects. The method makes successive demandsupon the host computer although the algorithms are quite general. Thissystem bears, again, no resemblance to the system of the presentapplication which does not use two-dimensional filters.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide an on-line dirt counter formeasuring and recording dirt speck sizes and intensities on a firstsurface of a web of moving paper.

It is a more specific object of the invention to provide such a counterwhich includes a light source disposed to reflect light off the firstsurface of the web of moving paper.

It is a still more specific object of the invention to provide such acounter wherein the reflected light is split by a beam splitter, one ofthe split beams being directed to an out-of-focus detector, the otherone of the spit beams being directed to an in-focus detector.

It is a still further object of the invention to provide such a counterwherein the reading of the sensor of the out-of-focus split beam issubtracted from the reading of the sensor of the in-focus split beam.

In accordance with the invention there is provided an on-line dirtcounter for measuring and recording dirt speck sizes and intensitieswhich appear on a first surface of a web of moving paper, comprising:

a light source spaced from said first surface of said web of movingpaper for directing a beam of light at said first surface of said web ofmoving paper, said beam of light being reflected from said first surfaceof said web of moving paper;

a beam splitter spaced from said first surface of said web of movingpaper to receive said reflected beam and to provide a first split beam,travelling in a first direction, and a second split beam, travelling ina second direction different from said first direction;

an in-focus sensor means placed to receive said first split beam and toprovide an in-focus signal having an in-focus magnitude proportional tothe magnitude of the light intensity of said first split beam;

an out-of-focus sensor means placed to receive said second split beamand to provide an out-of-focus signal having an out-of-focus magnitudeproportional to the magnitude of the light intensity of said secondsplit beam;

means for subtracting said out-of-focus magnitude from said in-focusmagnitude to provide a difference magnitude;

means for providing a threshold signal having a threshold valueproportional to a threshold intensity level;

comparator means for comparing said difference magnitude to saidthreshold magnitude; and

means for storing the occurrences and magnitudes of said differencemagnitude on all occasions when said difference magnitude is greaterthan said threshold magnitude.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be better understood by an examination of thefollowing description, together with the accompanying drawings, inwhich:

FIG. 1 is a perspective view of a dirt counter in accordance with theinvention;

FIG. 1A illustrates a possible arrangement for driving the housing in across-machine direction;

FIG. 2 is a schematic view of the data analysis board;

FIG. 3 is a flow chart of the process in accordance with the invention;and

FIG. 4 is a flow chart of an alternate process.

DESCRIPTION OF PREFERRED EMBODIMENTS

It is the purpose of the present invention to provide an on-line dirtcounter device to handle dirt speck detection and speck size estimationin the same manner as human judgment described in TAPPI countprocedures. A discussion of the principles involved, without adescription of specific apparatus or method steps, is given in"Emulating the TAPPI Dirt Count with a Microcomputer", Jordan et al,Journal of Pulp and Paper Science, Vol. 14, No. 1, January 1988, pps J16to J19.

For dirt speck detection, any potential specks are detected whenevertheir contrast to neighboring background is higher than a predeterminedthreshold. A contrast image can be derived from an original image byapplying a two-dimensional medium-pass filter. This is equivalent toextracting the difference between an original image and itscorresponding out-of-focus one. Using two camera, for example, chargecoupled device linear arrays, one viewing a paper portion in focus andthe other viewing the same portion, but partially out of focus, acontrast image can be obtained from the difference of the two lineararray signals. Any clusters of picture elements of the contrast imagewith their digitized values higher than a predetermined threshold areconsidered as dirt specks.

Integrated contrast values of the detected dirt specks are measured andstored. After a predetermined time interval, or a predetermined paperarea coverage, all stored dirt speck integrated values are retrieved,converted into corresponding sizes by a precalculated look-up table, andaccumulated in a dirt speck histogram. The look-up table can beconstructed from graphs similar to that shown at FIG. 3 of Jordan et alreferred to supra.

Accumulated statistical data and histograms can be reported regularly ina specific time interval until the paper roll is exhausted, or uponreceipt of an operator hault request. At this point, all data are resetfor preparing measurements of a new paper roll.

Referring to FIG. 1, the counter, illustrated generally at 1, comprisesa housing 3. Disposed within the housing is a light guide 5 which guideslight, as will be described below, to a focussing lens 7. After passingthrough the lens, the light is directed to a beam splitter 9 whichdirects one beam of light at an out-of-focus camera, for example, a CCD(charge couple device) sensor linear array 11 which is supported by aplate 13. The other split beam of light is directed at an in-focuscamera, for example, a CCD sensor linear array 15 which is supported ona plate 17.

The outputs of sensors 11 and 15 are fed to processor 19 via buses 21and 23 respectively.

Spaced from the housing 3 (in the illustrated example, underlying thehousing 3) are paper guiding means comprising spaced stabilizer plates25 and 27 having a space 29 between them. A web of paper to be testedmoves in the machine direction, illustrated by the arrow A, in the spacebetween the stabilizer plates. The means for guiding the paper isphysically connected to the housing, to move with the housing as will bedescribed below, by beams 31.

The housing 33 moves in a cross-machine direction, illustrated by arrowB, along the rail 33 using means well known in the art. For example, asillustrated in FIG. 1A, housing 3 can be attached to a roller 3R whichis guided for movement in the cross-machine direction by slot 33S inrail 33. The roller would be driven by a motor as is well known in theart.

A light source 35 is disposed to direct a beam of light 37 to onesurface of the web of moving paper (in the illustrated embodiment, thetop surface). The beam 37 is reflected by the surface of the web ofmoving paper to provide a reflected beam 39 which is received by thelight guide 5. The light guide 5 directs the beam 39 to the focussinglens 7 which in turn directs the focus beam to beam splitter 9.

Beam splitter 9 provides a first split beam 41, directed at theout-of-focus CCD sensor 11, and a second split beam 43, directed at thein-focus CCD sensor 15.

The output signals of the sensors 11 and 15 are transmitted to a dataanalysis board, which is not illustrated in FIG. 1, but which isinstalled in the processor 19.

Referring to FIG. 2, the data analysis board comprises a subtractor 45having its positive input terminal fed with the in-focus signal and itsnegative input terminal fed with the out-of-focus signal. The sum of thefocus signal less the out-of-focus signal is fed to analog-to-digitalconverter 47 whose output is fed to one input of a comparator 49. Thesecond input of the comparator 49 is fed a predetermined thresholdsignal.

The output of the comparator 49 is fed to an enable terminal ofaccumulator 51, and the output of the analog-to-digital converter 47 isfed, in parallel, to the data input terminal of accumulator 51. Theoutput of accumulator 51 is fed to the processor.

In operation, and referring first to FIG. 1, the dirt counter works asfollows:

The reflected light 39 is guided, by guide 5, to the lens 7 where it isfocussed and applied to beam splitter 9. The beam splitter 9 directs afirst split beam 41 to the out-of-focus sensor 11, and a second splitbeam, 43, to the in-focus sensor 15.

Turning now to FIG. 3, it can be seen that the two video signals aresynchronized by using the same data clock and exposure control signals.The outputs of the in-focus array and out-of-focus array are amplified,and the amplified signals are then converted to digital signals. Theout-of-focus digital value is then subtracted from the in-focus digitalvalue, and the difference signal is then compared with a programthreshold signal. If the difference signal exceeds the threshold signal,then a logical signal is fed to an accumulator to activate theaccumulator. The signal is also fed to an invigulator. The differencedigital signal is sent to the accumulator which, being enabled, computesthe sum of consecutive array elements of high difference values, i.e.,it performs the integration step.

When the difference signal falls to a value lower than the thresholdsignal, the invigulator will turn off the accumulator. The values in theaccumulator are stored in the processor memory array and an integratedintensity histogram as well as other statistical data of each measureddirt speck is formed. The histogram may be read at program timedintervals or at program paper area coverages.

The integrated values are then converted into their corresponding sizesusing a predetermined look-up table, and the size data is accumulated ina second accumulator. It is now possible to receive reports, forexample, histograms, in terms of size rather than in terms of integratedvalues.

As will be apparent, it is also possible to use the data to stop thepaper movement on detection of speck sizes exceeding predeterminedthresholds of, for example, size and intensity.

As can be seen, the FIG. 4 process is very similar to the FIG. 3 processexcept that the subtraction is performed while the in-focus andout-of-focus signals are still in their analog state. The differenceanalog signal is then compared with a program threshold analog signal.In all other respects, the process as illustrated in FIG. 4 is the sameas the process as illustrated in FIG. 3.

Although specific embodiments have been described, this was for thepurpose of illustrating, but not limiting, the invention. Variousmodifications, which will come readily to the mind of one skilled in theart, are within the scope of the invention as defined in the appendedclaims.

We claim:
 1. An on-line dirt counter for measuring and recording dirtspeck sizes and intensities which appear on a first surface of a web ofmoving paper, comprising:a light source spaced from said first surfaceof said web of moving paper for directing a beam of light at said firstsurface of said web of moving paper, said beam of light beingsubstantially diffusely reflected from said first surface of said web ofmoving paper; an in-focus sensor means including first focussing meansand a first detector placed at an in-focus position to receive a firstpart of said reflected beam emitted at an angle with respect to saidsurface, and to provide an in-focus magnitude signal having an in-focusmagnitude proportional to the magnitude of the light intensity of saidfirst part; an out-of-focus sensor means including second focussingmeans and a second detector having a same aperture as said firstdetector placed at an out-of-focus position to receive a second part ofsaid reflected beam emitted substantially at said angle with respect tosaid surface, and to provide an out-of-focus magnitude signal having anout-of-focus magnitude proportional to the magnitude of the lightintensity of said second part; means for subtracting said out-of-focusmagnitude from said in-focus magnitude to provide a differencemagnitude; comparator means for comparing said difference magnitude to athreshold magnitude; and means for storing the occurrences andmagnitudes of said difference magnitude on all occasions when saiddifference magnitude is greater than said threshold magnitude.
 2. A dirtcounter as defined in claim 1, further comprising a beam splitter spacedfrom said first surface of said web of moving paper to receive saidreflected beam of light substantially normally with respect to saidfirst surface and to split said reflected beam into said first and saidsecond parts.
 3. A dirt counter as defined in claim 2 wherein said meansfor storing comprises an accumulator having a signal input terminal anda control input terminal;said difference magnitude being fed to saidsignal input terminal; an enabling signal being fed to said controlterminal when said difference magnitude exceeds said thresholdmagnitude; whereby, said difference magnitude is stored in saidaccumulator when said difference magnitude exceeds said thresholdmagnitude.
 4. A dirt counter as defined in claim 3 and including meansfor calculating integrated intensities from said stored differencemagnitudes and for forming an integrated intensity histogram for eachdetected speck, indicative of the size and intensity of each speck, fromsaid integrated intensities.
 5. A dirt counter as defined in claim 4 andincluding look-up table means for converting said integrated intensitiesto corresponding sizes, and to form corresponding size histograms.
 6. Adirt counter as defined in claim 4 wherein said means for calculating,said means for forming histograms and said look-up table compriseprocessor means.
 7. A dirt counter as defined in claim 6 wherein saidfirst surface of said web of moving paper comprises the top surface ofsaid web of moving paper;said beam of light being reflected upwardlyfrom said top surface of said web of moving paper; and furtherincluding:light guide means disposed above said top surface of said webof moving paper to receive said reflected light beam; lens meansdisposed above said light guide means, said reflected beam beingdirected to said lens means to be focussed thereby; said lens meansdirecting said focus beam to said beam splitter.
 8. A dirt counter asdefined in claim 7 wherein said first direction is parallel to said topsurface of said web of moving paper;said second direction isperpendicular to said top surface of said web of moving paper;saidin-focus sensor means comprising a linear array charge couple device;and said out-of-focus sensor means comprising a linear array chargecouple device.
 9. A dirt counter as defined in claim 8 and furthercomprising;a first top stabilizer plate; a second bottom stabilizerplate defining a space between said first and second stabilizer plates;said web of moving paper moving through said space between saidstabilizer plates; an opening in said top stabilizer plate through whichsaid beam of light from said light source is directed at said topsurface of said web of moving paper.
 10. A dirt counter as defined inclaim 9 wherein said in-focus sensor means is supported by an in-focussensor means support plate, and wherein said out-of-focus sensor meansis supported by an out-of-focus sensor means support plate.
 11. A methodfor measuring and recording dirt speck sizes and intensities whichappear on a first surface of a web of moving paper, comprising:directinga beam of light from a light source at said first surface of said web ofmoving paper, said beam of light being reflected substantially diffuselyfrom said first surface of said web of moving paper; focusing andreceiving a first part of said reflected beam emitted at an angle withrespect to said surface in an in-focus sensor means placed at anin-focus position to provide an in-focus magnitude signal having anin-focus magnitude proportional to the magnitude of the light intensityof said first part of said beam; focusing and receiving a second part ofsaid reflected beam emitted substantially at said angle with respect tosaid surface in an out-of-focus sensor means having a same aperture assaid in-focus sensor means and being placed at an out-of-focus positionto provide an out-of-focus magnitude signal having an out-of-focusmagnitude proportional to the magnitude of the light intensity of saidsecond part of said beam; subtracting said out-of-focus magnitude fromsaid in-focus magnitude to provide a difference magnitude; comparingsaid difference magnitude to a threshold magnitude; and storing theoccurrences and magnitudes of said difference magnitudes on alloccasions when said difference magnitude is greater than said thresholdmagnitude.
 12. A method as defined in claim 11, further comprising astep of splitting said reflected beam of light in a beam splitter toprovide said first part of said beam and said second part of said beamrespectively, said beam splitter receiving said beam substantiallynormally with respect to said first surface.
 13. A method as defined inclaim 12 wherein the stored occurrences of said difference magnitude arestored in an accumulator which has a signal input terminal and a controlinput terminal;the difference magnitude being fed to the signal inputterminal; an enabling signal being fed to the control input terminalwhen the difference magnitude exceeds the threshold magnitude; whereby,said difference magnitude is stored in said accumulator when saiddifference magnitude exceeds said threshold magnitude.
 14. A method asdefined in claim 13 and including the step of calculating integratedintensities from said stored difference magnitudes and for forming anintegrated intensity histogram for each detected speck, indicative ofthe size and intensity of each speck, from said integrated intensities.15. A method as defined in claim 14 and including the step of convertingsaid integrated intensities to corresponding sizes using look-up tablemeans, and to then forming corresponding size histograms.
 16. A methodas defined in claim 15 wherein said calculating, said histogram forming,and said look-up table means step are performed in processor means. 17.A method as defined in claim 16 wherein said first surface of said webof moving paper comprises the top surface of said web of movingpaper;said beam of light being reflected upwardly from said top surfaceof said web of moving paper; said beam of light being guided by lightguide means above said top surface of said web of moving paper to lensmeans diposed above said light guide means, said lens means focussingsaid beam of light; said focussed beams being directed by said lensmeans to said beam splitter.
 18. A method as defined in claim 17 whereinsaid web of moving paper moves through a space between stabilizer platesand is guided by said stabilizer plates.